Mobile phase— Prepare a suitable filtered and degassed mixture of 0.1 M monobasic sodium phosphate and methanol (65:35). Make adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Labetalol Hydrochloride RS in Mobile phase to obtain a solution having a known concentration of about 0.5 mg per mL.

Resolution solution— Dissolve a quantity of methylparaben in the Standard preparation to obtain a solution containing about 0.08 mg per mL.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 50 mg of labetalol hydrochloride, to a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 20-cm column that contains packing L1 and is maintained at 60 ± 1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the column efficiency determined from the analyte peak is not less than 700 theoretical plates; the tailing factor for the analyte peak is not more than 2.0; and the relative standard deviation for replicate injections is not more than 1.5%. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the relative retention times are about 0.6 for methylparaben and 1.0 for labetalol; and the resolution, R, between the methylparaben and labetalol is not less than 2.0.

Procedure— Separately inject equal volumes (about 5 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the area responses for the major peaks. Calculate the quantity, in mg, of labetalol hydrochloride (C19H24N2O3·HCl) in each mL of the Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Labetalol Hydrochloride RS in the Standard preparation; V is the volume, in mL, of Injection taken; and rU and rS are the peak area responses obtained from the Assay preparation and the Standard preparation, respectively.